Wireless communication systems have recently developed to radio data packet communication systems that can provide multimedia services with a high speed and high quality beyond the early voice-oriented services. To transmit data with a high speed by using a limited frequency resource, the wireless communication system uses Adaptive Modulation and Coding (AMC) that regulates a modulation rate and a coding rate according to a channel state. In this case, a channel of the wireless communication system changes frequently due to several factors, such as, white noise, received signal power change caused by fading, shadowing, Doppler effect depending on movement of a mobile terminal and frequent speed change, interference caused by other users or multi-path signals, etc.
As described above, the channel of the wireless communication system changes according to a state of a radio resource. That is, a signal transmitted by a transmitter of the wireless communication system is distorted according to changes in the channel. To compensate for the distortion of the signal, a receiver of the wireless communication system performs channel estimation. For example, when the wireless communication system uses Binary Phase Shift Keying (BPSK) and Quadrature PSK (QPSK), the receiver compensates for the channel by predicting only a phase of a received signal. The BPSK and the QPSK are low-order modulation schemes included in the AMC. When the wireless communication system uses a high-order modulation scheme such as 8-ary Phase Shift Keying (8PSK) and 16-ary Quadrature Amplitude Modulation (16QAM), a plurality of symbols are located in each quadrant and a plurality of symbols each having a different amplitude component can be located at the same phase. Therefore, the receiver needs to estimate not only a phase component but also an amplitude component.
The transmitter transmits a pilot signal in order to estimate the channel. The pilot signal is known to the receiver in advance. That is, the receiver estimates the channel by using the pilot signal transmitted by the transmitter, and thereafter demodulates a data signal by using a value of the estimated channel. To improve accuracy of the channel estimation in the receiver, the transmitter has to transmit more amounts of pilot signals more frequently.
Since the pilot signal and the data signal use limited resources (e.g., frequency and time resources) in the transmitter, a throughput for the data signal decreases when the pilot signal is transmitted more frequently.
Accordingly, by considering a Doppler frequency, delay spread, etc., the transmitter transmits the pilot signal with a specific interval in a frequency and time resource as shown in FIG. 1.
FIG. 1 illustrates a pilot pattern of a conventional Orthogonal Frequency Division Multiplexing (OFDM) system.
Referring to FIG. 1, the OFDM system transmits a pilot signal with a specific interval in a frequency resource and a time resource.
By using the pilot signal transmitted with the specific interval, a receiver calculates a channel value of a data signal region, that is, a region where the pilot signal is not transmitted. For example, in the estimation of the channel value of the data signal region, the receiver may perform linear interpolation or the like on the channel value estimated using the pilot signal.
When a signal is transmitted and received using the same pilot pattern in the OFDM system, a preamble with a high pilot density has a best channel estimation throughput. Channel estimation information is used in channel estimation for subsequent OFDM symbols. In this case, there is a problem in that the greater the temporal distance from the preamble, the poorer the channel estimation throughput.
The aforementioned problem can be addressed by regulating a number of times of transmitting the pilot pattern by using feedback information of a mobile terminal. However, there is still an unsolved problem in that the channel estimation throughput deteriorates as a result of uniform density of pilot signals included in a frame. That is, when subcarriers are located at edge portions in a two-dimensional downlink frame, a relatively small number of pilots can be used in channel estimation, and thus there is a problem in that a link throughput decreases.
As described above, in the conventional method, the same modulation scheme and coding rate are used at all locations within a frame according to channel quality information in a frame unit. When channel estimation is performed on a rear portion of the frame, spectrum usage efficiency may deteriorate. This is because a link throughput is relatively good in a front portion, but a higher modulation scheme and coding rate are not used in the front portion of the frame. On the contrary, when channel estimation is performed on the front portion of the frame or on edge portions of the frame wherein the rear portion has a poor link throughput, errors may frequency occur in the rear portion of the frame.